1. Field of the Invention
The present invention relates to a vacuum heating apparatus, and more particularly to an electron impact (collision) heating device in which a thermo-electron emitted from a filament by an acceleration voltage is caused to collide with a heater to generate heat.
2. Related Background Art
A semiconductor production technique often requires a process of rapidly heating a semiconductor substrate. Particularly, activation annealing of a wide band gap semiconductor such as silicon carbide (SiC) requires high temperature of about 2000° C.
As an example of a vacuum heating apparatus, an electron impact heating device has been proposed in which a thermo-electron emitted from a filament by an acceleration voltage applied between the filament and a vacuum chamber is caused to collide with a heater to generate heat (Japanese Patent No. 2912613, Japanese Patent No. 2912616, and Japanese Patent No. 2912913).
In the vacuum heating apparatus, an aluminum vacuum chamber is used having high thermal conductivity and low emissivity.
However, the aluminum vacuum chamber has a soft flange portion for joining members that constitute the vacuum chamber, and thus use of a metal gasket is difficult and an ring seal of fluororubber or resin is used for a vacuum seal. Meanwhile, as described above, an activation annealing process for electrically activating a silicon carbide (SiC) substrate into which an impurity is ion-implanted requires very high temperature of about 2000° C.
Specifically, an amount of thermo-radiation to the vacuum chamber is significantly increased as compared with in an annealing process with thermo-radiation from a heater of about 1000° C. that has been used in a conventional silicon (Si) device.
As described above, with a structure in which an O ring is simply held as usual in a very high temperature region of 2000° C., radiation heating during heat treatment reduces exhaust properties due to deterioration of the O ring, and also reduces annealing properties.
Specifically, the vacuum heating apparatus was assembled to release gas for heating, and then a sample of a silicon carbide (SiC) epitaxial substrate into which aluminum was implanted at, for example, 500° C. at a concentration of 2.0×1018/cm3 was annealed at 2000° C. for 10 min.
In this case, maximum pressure of the vacuum chamber during annealing is 3.6×10−4 Pa, and when surface flatness of a sample surface after annealing was measured by an atomic force microscope (AFM), very good flatness with an RMS (Root-Mean-Square) value of 0.6 nm was obtained.
However, when the sample was annealed at 2000° C. for 10 min to (?) 1000 times, and then the same sample was annealed at 2000° C. for 10 min, the maximum pressure of the vacuum chamber during annealing was 7.6×10−3 Pa, and the degree of vacuum was reduced. When surface flatness of the sample surface after annealing was measured by the atomic force microscope (AFM), there was a rough surface with an RMS value of 3.5 nm.
Further, when ultimate pressure of the vacuum chamber was measured, it was recognized that the pressure was 4.3×10−4 Pa and vacuum properties were reduced by about two digits. Specifically, a conventional O ring seal in which two parallel seal surfaces are placed to face each other and an O ring made of fluororubber or resin is placed and held in a groove provided between the seal surfaces directly receives radiation from heating means for heating up to around 2000° C.
Thus, the O ring is deteriorated in a short time to release gas, thereby reducing the vacuum properties of the vacuum chamber. This also reduces flatness of the substrate surface during annealing.
The present invention has an object to prevent deterioration of an O ring due to radiation heating in a vacuum heating apparatus, and allow heat treatment of a substrate with good annealing properties.